What is the optimal course of action to recover the remaining amount from a procrastinator after I lost half of my capital due to the devaluation of the local currency and the rise in the price of the dollar and gold?

The summary of the matter is that there is no way but for you to receive the remaining amount in the local currency, and give him his checks and car contract. You will be rewarded by Allah, unless he wishes to donate something extra to you out of good repayment, according to the saying of the Prophet, peace and blessings be upon him: "Indeed, the best among you are those who are best in fulfilling obligations."

However, he is not obligated to pay more than the original amount in pounds, and the fluctuation of its price against the dollar or gold is not considered. The consideration in repaying fixed debts in a certain currency is by its equivalent, not by its value. It is not permissible to link fixed debts to the price level. It is also not permissible to agree to record the debt in the debtor's liability as equivalent to the value of that currency in gold or another currency.